Counterion-induced attraction between rigid polyelectrolytes

We report on results of long-time Brownian-dynamics simulations of electrostatic interactions between two rigid polyelectrolyte rods. We find that the interaction can be both repulsive, as obtained from mean-held theory, and attractive. The onset of attraction depends not only on the fixed charge density of the rod, but also on its radius. The attractive force is found to be due to the development of positional correlations between the counterions condensed on the two rods, for which we propose a simple analytical model.